Fuel cell systems are seen as a promising alternative to traditional power generation technologies due to their low emissions, high efficiency and ease of operation. Fuel cells operate to convert chemical energy into electrical energy. Proton exchange membrane fuel cells comprise an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. In a catalyzed reaction, a fuel such as hydrogen, is oxidized at the anode to form cations (protons) and electrons. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. The electrons cannot pass through the membrane and are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts at the catalyst layer, with electrons returned from the electrical circuit, to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction product.
Proton exchange membranes require a wet surface to facilitate the conduction of protons from the anode to the cathode, and otherwise to maintain the membranes electrically conductive. It has been suggested that each proton that moves through the membrane drags at least two or three water molecules with it (U.S. Pat. No. 5,996,976). U.S. Pat. No. 5,786,104 describes in more qualitative terms a mechanism termed “water pumping”, which results in the transport of cations (protons) with water molecules through the membrane. As the current density increases, the number of water molecules moved through the membrane also increases. Eventually the flux of water being pulled through the membrane by the proton flux exceeds the rate at which water is replenished by diffusion. At this point the membrane begins to dry out, at least on the anode side, and its internal resistance increases. It will be appreciated that this mechanism drives water to the cathode side, and additionally the water created by reaction is formed at the cathode side. Nonetheless, it is possible for the flow of gas across the cathode side to be sufficient to remove this water, resulting in drying out on the cathode side as well. Accordingly, the surface of the membrane must remain moist at all times. Therefore, to ensure adequate efficiency, the process gases must have, on entering the fuel cell, a predetermined or set relative humidity and a predetermined or set temperature which are based on the system requirements.
A further consideration is that there is an increasing interest in using fuel cells in transport and like applications, e.g. as the basic power source for cars, buses and even larger vehicles. As compared to some stationary applications, this presents some unique requirements. More particularly, it is necessary that the power delivered by a fuel cell be capable of rapid change between different power levels, and these power levels can be quite different. Thus, in urban driving, it is common for fuel cells to be required to frequently switch between minimum, or even zero power, to a maximum power level and back again. Maintaining appropriate humidity levels under such severe operating conditions is not easy. Additionally, a fuel cell must be capable of providing this functionality under a wide range of ambient air conditions.
Accordingly, in this art one can find numerous proposals for maintaining humidity in fuel cell systems. One conventional way to humidify a gas stream is to pass a gas as a stream of fine bubbles through water. As long as the process gas has sufficient contact time with the water, controlling the temperature of the water controls the amount of water in the gas stream. However, these bubble column type humidifiers are generally not suitable for fuel cells. The humidifiers tend to be large and costly. Moreover, the humidifiers are unable to react fast enough to meet the load following requirements of the fuel cell system. As a result, at high gas flow rates the system becomes unstable, unreliable and unresponsive. In addition, this humidification system never reaches 100% relative humidity in practice and this limits the flexibility or adaptability of the system.
In some prior art fuel cells, incoming process gases are humidified by flowing each gas on one side of a water vapor exchange membrane and by flowing deionized water on the opposite side of the membrane. In this way, water is osmotically transferred across the membrane to the fuel and oxidant gases. However, these systems have process parameter restraints that cause problems and inefficiencies when used in conjunction with fuel cells. Since the membrane is at the same temperature as the fuel stack, there is no independent control of the relative humidity or temperature of the process gases and thus the system is limited in its ability to adjust to different situations.
Other humidification methods include exposing the incoming process gas to a source of steam or metering in a quantity of fine water droplets into the gas supply line (U.S. Pat. No. 5,432,020). However, in the past, these systems tended to be large, complex, slow acting, and possessed inadequate dynamic controllability.
There remains a need for a humidifier that can offer rapid dynamic control, as well as precise and accurate temperatures and relative humidities for incoming fuel cell process gases. More particularly, such a humidifier should enable relative humidity and temperature to be controlled independently over a wide variety of flow rates, for both the oxidant and fuel systems.